FIG. 4 is an equivalent circuit diagram of a transmission filter of a conventional antenna duplexer described in WO 01/005031. The conventional antenna duplexer includes an antenna terminal, transmission filter 1 connected to the antenna terminal, and a reception filter connected to the antenna terminal. A pass band of the reception filter is lower than that of the transmission filter.
Transmission filter 1 includes input terminal 2, series resonator 3 connected to input terminal 2, series resonator 4 connected to an output port of series resonator 3, series resonator 5 connected to an output port of series resonator 4, and output terminal 6 connected to an output port of series resonator 5. Transmission filter 1 also includes parallel resonator 7 having one port connected between series resonator 3 and series resonator 4, parallel resonators 8 and 9 having respective one port connected between series resonator 4 and series resonator 5. Transmission filter 1 further includes inductance elements 10 and 11. Inductance element 10 s electrically connected between ground 12 and a node at which another port of resonator 7 and another port of resonator 8 are connected. Inductance element 11 is connected between ground 12 and another port of parallel resonator 9.
In transmission filter 1, parallel resonator 7, parallel resonator 8 and inductance element 10 produce an attenuation pole at a frequency lower than a transmitting frequency band. Parallel resonator 9 and inductance element 11 produce an attenuation pole at a frequency higher than a transmitting band width.
Transmission filter 1 is a ladder type filter. In one parallel arm, parallel resonator 9 having small capacitance COP3 and inductance element 11 having inductance LSER2 produce an attenuation pole at frequency F1 higher than the pass band that is the transmission band width of the transmission filter.F1=1/{π(LSER2COP3)1/2}
Frequency F is raised to increase an attenuation at a frequency higher than the transmission frequency band, thereby suppressing harmonics contained in the transmission signal input from the input terminal.
An LC series resonant circuit composed of parallel resonator 9 and inductance element 11 has impedance Z1.Z1=jωLSER2+1/(jωCOP3)
In transmission filter 1, since capacitance COP3 of parallel resonator 9 is small, impedance Z1 is high at the frequency higher than the transmission frequency band. Consequently, the conventional antenna duplexer hardly obtains a sufficient amount of attenuation at the frequency higher than the transmission frequency band, accordingly being prevented from sufficiently suppressing a spurious caused by harmonics contained in the transmission signal.